Page:Popular Science Monthly Volume 6.djvu/402

386 meridian of the star to that of the comet. To make this measure, it is customary to fix in the focus of the telescope some uneven number of fine filaments of spider's-web at (say) equal distances apart, and to allow the telescope to remain fixed while the diurnal rotation of the earth carries the body first to be observed into the field of the telescope and slowly across this. As it crosses each of the threads, the time at which it is exactly on the thread is noted. Now, when the second body enters the field of the telescope (which is supposed to remain fixed in its former position) the times of its passage over the various threads are noted.

The mean of the times for the first body gives the time at which this body was on the middle thread (these being at equal intervals), while the mean of the times for the second body gives the corresponding time for the second body, and the difference of these two times gives evidently the distance which one of them is, east or west, of the other, expressed in time. This may be easily reduced to degrees, etc., by the rule that twenty-four hours is equal to 360 degrees.

If it were possible for an astronomer to note the exact instant of the transit of a star over a thread, it is plain that one thread would be sufficient; but, as all estimations of this time are, from the very nature of the case, but approximations, several threads are inserted in order that the accidental errors of estimations may be eliminated, as far as possible. The method of making these estimations will be better understood from the two following figures, 1 and 2. Fig. 1 represents



the reticle of a transit-instrument as it would be viewed by an observer, where twenty-five threads are placed arranged in groups or tallies of five. The star may enter on the left hand in the figure, and may be supposed to cross each of these wires, the time of its transit over each